Universal Mechanism of Band-Gap Engineering in Transition-Metal Dichalcogenides

Kang, Mingu; Kim, Beomyoung; Ryu, Sae Hee; Jung, Sung Won; Kim, Jimin; Moreschini, Luca; Jozwiak, Chris; Rotenberg, Eli; Bostwick, Aaron; Kh, Kim

doi:10.1021/acs.nanolett.6b04775

Cited by 193 publications

(163 citation statements)

References 34 publications

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“…We note a similar work by another group published after we finished our work 40 . They performed ARPES on MoS 2 , MoSe 2 , MoTe 2 , WS 2 and WSe 2 with Rb dosing and found that only MoTe 2 showed the indirect to direct band gap transition.…”

Section: Resultssupporting

confidence: 58%

Possible electric field induced indirect to direct band gap transition in MoSe2

Kim

Kyung

Seo

et al. 2017

Sci Rep

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Direct band-gap semiconductors play the central role in optoelectronics. In this regard, monolayer (ML) MX 2 (M = Mo, W; X = S, Se) has drawn increasing attention due to its novel optoelectronic properties stemming from the direct band-gap and valley degeneracy. Unfortunately, the more practically usable bulk and multilayer MX 2 have indirect-gaps. It is thus highly desired to turn bulk and multilayer MX 2 into direct band-gap semiconductors by controlling external parameters. Here, we report angle-resolved photoemission spectroscopy (ARPES) results from Rb dosed MoSe 2 that suggest possibility for electric field induced indirect to direct band-gap transition in bulk MoSe 2 . The Rb concentration dependent data show detailed evolution of the band-gap, approaching a direct band-gap state. As ionized Rb layer on the surface provides a strong electric field perpendicular to the surface within a few surface layers of MoSe 2 , our data suggest that direct band-gap in MoSe 2 can be achieved if a strong electric field is applied, which is a step towards optoelectronic application of bulk materials.Whether a semiconductor has a direct or indirect band-gap greatly affects its optical properties; excitons in direct band-gap semiconductors, for example, strongly couple to photons. For such reason, direct band-gap semiconductors can be used for optoelectronic applications such as light emitting and laser diodes 1, 2 . Monolayer 2H-MX 2 (M = Mo, W; X = S, Se) also have a direct band-gap and have drawn considerable attention due to their novel electronic properties as described by the Massive Dirac-Fermion model 3,4 . The coupling between the valley degree of freedom and circularly polarized light opened the field of valley Hall effect 5,6 . Such intriguing physics can be realized only in 1 ML 2H-MX 2 because multi-layer 2H-MX 2 has an indirect band gap and thus does not strongly couple to light. Photoluminescence and photo absorption spectroscopy experiments indeed show that photon-exciton coupling is strong only in 1 ML 2H-MX 2 7-10 .Making bulk MX 2 a direct gap material would be highly desirable because the necessity for high quality ML MX 2 puts severe limitation on the actual application. There were many computational or experimental studies in search of a way to control the band structure and gap size in bulk 2H-MX 2 . It was found that the band gap size of TMDs can be modified by using various methods such as strain [11][12][13][14][15][16][17] , chemical doping [18][19][20][21][22] , electric field 12, 23-26 , making heterostructures 12, 27-31 and using different substrates for thin films [32][33][34][35] . However, there has not been any proposal to induce direct band gap in bulk MX 2 .Alkali metal atoms evaporated on the surface of a sample not only dope electrons to the sample but also generate a strong electric field near surface 36 . Our strategy to the issue of inducing a direct gap is to apply a strong electric field perpendicular to the MoSe 2 layers by using such alkali metal dosing. Valence band dispersions...

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Section: Resultssupporting

confidence: 58%

Possible electric field induced indirect to direct band gap transition in MoSe2

Kim

Kyung

Seo

et al. 2017

Sci Rep

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“…Experimental observations confirmed a variation in band gap in the range of 0.8-2.0 eV, which covers a wide spectral range from visible to near infrared, with a tendency from indirect to direct BGs. These results confirmed surface Stark effect as a universal mechanism of band-gap engineering on the basis of the strong 2D nature of van der Waals semiconductors [72].…”

Section: Some Recent Findingssupporting

confidence: 73%

Band Gap Engineering of Hybrid 2-D Nanomaterials Some Recent Developments

Ahmad¹

2017

MOJPS

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Successful modifications of band gaps of bulk alloy semiconductors (i.e. in binary, ternary and quaternary compounds) for electronic and optoelectronic device applications encouraged the researchers to explore similar strategies in 2D-materials involving graphene and graphene like transition metal chalcogenides and other layered materials. Being atomically thin layers, there are numerous other possibilities to explore in these 2D-materials involving lateral, vertical heterostructure formations besides substitution, and doping of other atomic species to fix their band gaps somewhere between zero of the graphene and the wide band gap, for example, of h-BN (i.e. 6eV band gap) in hybrid type h-BNC lattice. The recent developments taking place in this important area of research in band gap engineering of 2D-hybrid materials is briefly discussed in this review.

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“…This can either be a single-particle effect 28 or a combination of single-and many-body effects 25 , the latter of which suggests a negative electronic compressibility (NEC), the motion of the chemical potential μ towards the VBM, i.e. dμ/dN < 0.…”

mentioning

confidence: 99%

Giant spin-splitting and gap renormalization driven by trions in single-layer WS2/h-BN heterostructures

et al. 2018

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In two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs), new electronic phenomena such as tunable bandgaps 1-3 and strongly bound excitons and trions emerge from strong many-body effects [4][5][6] , beyond the spin and valley degrees of freedom induced by spin-orbit coupling and by lattice symmetry 7 . Combining single-layer TMDs with other 2D materials in van der Waals heterostructures offers an intriguing means of controlling the electronic properties through these many-body effects, by means of engineered interlayer interactions [8][9][10] . Here, we use micro-focused angle-resolved photoemission spectroscopy (microARPES) and in situ surface doping to manipulate the electronic structure of single-layer WS 2 on hexagonal boron nitride (WS 2 /h-BN). Upon electron doping, we observe an unexpected giant renormalization of the spin-orbit splitting of the single-layer WS 2 valence band, from 430 meV to 660 meV, together with a bandgap reduction of at least 325 meV, attributed to the formation of trionic quasiparticles. These findings suggest that the electronic, spintronic and excitonic properties are widely tunable in 2D TMD/h-BN heterostructures, as these are intimately linked to the quasiparticle dynamics of the materials [11][12][13] . Coulomb interactions in 2D materials are several times stronger than in their 3D counterparts. In 2D TMDs, this is most directly evidenced by the presence of excitons with binding energies an order of magnitude higher than in the bulk 4 . Although the excitons in these 2D materials have been widely studied by optical techniques 13 , the impact of strong electron-electron interactions on the quasiparticle band structure remains unclear. Theory predicts that many-body effects will influence the spin-orbit splitting around the valenceband maximum (VBM) and conduction-band minimum (CBM) 14 . Although these should be observable by ARPES, a direct probe of many-body effects 15 , measurements so far have mainly focused on the layer-dependence of the single-particle spectrum and the direct bandgap transition in 2D TMD systems, including epitaxial single- . Unfortunately, the lateral size of mechanically assembled heterostructures is usually of the order of 10 μ m, much smaller than the beam spot of typical ARPES setups (≳ 100 μ m). Furthermore, sample charging on insulating bulk h-BN substrates would complicate ARPES experiments.We overcome these challenges as follows. We realize a high-quality 2D semiconductor-insulator interface by mechanically transferring a relatively large (~100 μ m) single-layer WS 2 crystal onto a thin flake of h-BN that has itself been transferred onto a degenerately doped TiO 2 substrate, as depicted in Fig. 1a. Sample charging is avoided because there is electrical contact from the continuous single-layer WS 2 flake to both the h-BN and the conductive TiO 2 . Figure 1b is an optical microscope image of the sample, including a flake of h-BN, approximately 100 μ m wide, surrounded by several transferred flakes of single-layer WS 2 on the Ti...

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Universal Mechanism of Band-Gap Engineering in Transition-Metal Dichalcogenides

Cited by 193 publications

References 34 publications

Possible electric field induced indirect to direct band gap transition in MoSe2

Possible electric field induced indirect to direct band gap transition in MoSe2

Band Gap Engineering of Hybrid 2-D Nanomaterials Some Recent Developments

Giant spin-splitting and gap renormalization driven by trions in single-layer WS2/h-BN heterostructures

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